Method of making metal drawing dies



Patented Aug. 9, 1949 UNITED STATES PATENT OFFICE METHOD OF MAKING METAL DRAWING DIES No Drawing. Application August 20, 1945, Serial No. 611,705

1 Claim. l This invention relates to improvements in forming dies for metal. It relates particularly to afiiie and means for making the same of zinc base a y.

The principal object of my invention is to provide a die for cold forming metal parts which is cheaper to manufacture than a steel die, but more durable and wear resistant than conventional zinc base alloy dies.

The process of cold forming of metal parts by means of dies consists of causing a permanent deformation by applying stresses above the elastic limit of the material being formed.

The forming is done by dies made of a variety of alloys, such as: steel, cast iron, bronze and zinc base composition.

The material used for dies must possess the following properties:

. Low 'coemcient of friction.

. Resistance to wear.

. Ability to retain the lubricating film.

. Ability to resist local deformation under compressive loads.

Zinc base alloys for forming dies because of their low initial cost, have found their great application in the stampings of aircraft parts and other parts of somewhat limited production.

Experience has shown that the life of zinc base forming dies is dependent on the ability of the material to retain its size and shape, that is to say the ability of edges, covers and protruding areas to resist repeated stresses.

It is known in the art that chromium plating of die surfaces results in greater die life. However, chromium plating of zinc base forming dies has not been possible owing to the fact that the electrolytes available do not permit the deposition of a continuous, adhering, smooth chromium layer.

It may be possible to prepare the zinc base alloy by plating copper and/ or nickel followed by polishing or bufling, but the cost is excessive and the process is not commercially feasible. It results also in an unsatisfactory product because of the multiplicity of metallic layers of different mechanical properties react differently upon application of compressive loads; hence, separation or peeling invariably takes place.

In the copending application of Russell Raab, Serial No. 604,093, filed July 9, 1945, the composition of a new electrolytic bath for the deposition of chromium is disclosed. By the use of this new and improved solution, I am able to deposit chromium directly on zinc base die forming alloy.

In my discovery, I have found that the surface rAwMH of the die requires no special preparation beside the customary removal of dirt, grease or foreign matter. This is accomplished by any accepted chemical and/or electrolytic process.

Cleaning is followed immediately by plating in an electrolyte made up in an aqueous solution as follows: chromium trioxide (Cr 03), 36-60 oz. per gal; tartaric or malic acid .001.25 oz. per gal; hydrofluoric acid .5-2.0 oz. per gal.

In practice, I have discovered that the chromium deposited on the zinc base forming dies does not require any polishing or buffing and can be put to use in the as plated condition.

Regular production runs on forming of steel and aluminum parts have shown that zinc base forming dies, chromium plated by my process, retain dimensional accuracy, do not sever or gall and their life is extended by 6 to 12 times their normal life.

The thickness of the chromium deposited should be no less than .0003" and not more than .010". For best results, I have found that the chromium layer should be from .0005" to .005" in thickness.

The alloy to which this process applies, has the following nominal composition:

Percent Zinc -93 Aluminum 3 /z-4/.- Copper 2 /23% Magnesium Up to 1% This alloy and others of substantially similar composition may be purchased in pigs on the open market under various trade names such as for example Kirkslte. In the commercial alloys thus obtained, small amounts of other metals are sometimes present. Some of these may be purposely added or may be present because it is not economical to further refine the base metals. I have found that the presence of these other metals does not materially interfere with the plating nor of the effect thereof on the die.

It will thus be seen that I have disclosed a method and means for making cheap metal forming dies which have relatively longer life than those now commercially available. In practice, I have, by plating as herein described, increased the life of zinc base alloys dies in some instances by twelve times.

Heretofore these soft metal dies have had a very short life even when used for forming soft metals such as aluminum and magnesium, and consequently were used only for special short production runs of work of the custom variety. My improved plated die is suitable for short production runs and is especially suitable for use in the manufacture of aircraft sections, automobile bodies of the expensive type where use of steel dies is prohibited by their high cost, etc.

My process is also applicable for restoring worn dies. Zinc base dies characteristically rail after short peroiods of use because the metal flows and the die loses its shape, and/or the metal cracks and chips at the corners. After cleaning, a worn die can be immersed in the above described electrolyte and plated back to size if it is not too badly damaged. In cases where the die has been originally plated, no stripping is necessary as it is possible to platedirectly on the existing plate with my process.

While specific examples have been given for purposes oi illustration, I wish it to be understood that my invention in its broader aspects is not limited to the examples given but only as defined in the claim appended below.

I claim:

The method of making metal drawing dies comprising casting the die of zinc base alloy to approximate size and shape then electrodepositing chromium upon the surface of the die in an elec trolyte of the following composition: chromium g5 553,265

4 trioxide, 36-60 oz. per gal. of water; tartaric acid, poi-.25 oz. per gal. of water; hydrofluoric acid 5-2.0 oz. per gal. of water.

CARL D. MACPHERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Great Britain May 14, 1943 

